Hyperammonemia and hepatic encephalopathy are generally believed to have the following pathology.
When nitrogenous compounds such as amino acids, amines and purine/pyrimidine bases are metabolized in organs in the living body, ammonia is produced. Besides the ammonia produced in this metabolic process, amino acids produced by digestion and decomposition of dietary protein are also converted to ammonia after absorption into the mucosa of the small intestine and then released into the portal vein. The ammonia generated by enterobacteria in the colon is also absorbed. Hence, the intestine plays a major role in the behavior of blood ammonia.
Since ammonia is a toxic substance, every organ has a metabolic mechanism that detoxifies and processes ammonia. In organs other than the liver, two reactions occur, one involving glutamic dehydrogenase to synthesize glutamic acid by incorporating ammonia into (α-ketoglutaric acid and the other for converting the produced glutamic acid to glutamine by binding with ammonia.
In the liver, ammonia is actively processed in the urea cycle. In normal state, ammonia metabolism is strictly regulated and the blood ammonia level is maintained at constant level. However, if some part of the ammonia detoxifying mechanism becomes abnormal or if ammonia detoxification and processing are not fully functional due to hepatic insufficiency or other cause, the blood ammonia increases to cause manifestation of hyperammonemia. If the protein uptake increases, more urea is produced in the liver and so is the urea that is secreted into the upper digestive tract. As a result, the production of ammonia from urea by enterobacteria through urease reaction increases to elevate the blood ammonia level.
Hepatic insufficiency is one of the typical diseases that cause hyperammonemia and the encephalopathy that is involved is called hepatic encephalopathy. If the ammonia level in cells increases, α-ketoglutaric acid which is located in the citric acid (TCA) cycle reacts with ammonia to form glutamic acid which further reacts with one molecule of ammonia to become glutamine. This reaction consumes ATP and the decrease in α-ketoglutaric acid impairs the turnover of the TCA cycle; as a result, the net ATP production decreases. The impaired metabolism is noticeable in the brain stem and adversely affects the function of the brain stem reticular formation which is important in the maintenance of consciousness level, thus causing disturbances of consciousness [Akiharu Watanabe, Rinsho Kanfuzengaku (Clinical Study of Hepatic Insufficiency), pp. 26-33, Nagai Shoten, 1994].
With the progress of hepatic insufficiency, the blood urea decreases and ammonia increases. In the urine, urea nitrogen decreases and the proportions of ammoniacal nitrogen, amino nitrogen, etc. in the total urinary nitrogen increase markedly. The liver is an organ having extremely high performance in reserve and its ability to synthesize urea changes little even if 80-90% of it is excised; hence, the increase of blood ammonia is not probably due to the lowering of the urea synthesizing ability; rather, a short circuit between the portal vein and the systemic circulation is formed in consequence of impairment of the hepatic parenchyme and ammonia is directly carried into the systemic circulation via this alternative pass way without passing through the liver [Taira Sakaguchi, Kanshikkan to Tanpakushitsu Taisha, Yakugaku Ryoiki no Byotai Seikagaku (Hepatic Disease and Protein Metabolism —Biochemistry of Pathology in Pharmacy), Hirokawa Shoten, pp. 152-155, 1976].
As hepatic insufficiency worsens, the blood ammonia level increases, whereupon psychoneurotic symptoms appear. In the early period, the patient is declined orientation, attention and concentration and fall into a clouding of consciousness and coma with the progress of the disease. In the later period, tremor and flapping involuntary movements (asterixis) occur in the superior limbs. In EEG, periodic and synchronous characteristic waveform patterns called “three-phase waves” appear [Igaku Daijiten (Encyclopaedia of Medicine), 18 ed., Nanzando, 1998].
The basics of therapy against hyperammonemia lie in suppressing the production of ammonia while promoting the detoxification and processing of ammonia. A most effective way to suppress the production of ammonia is reducing dietary protein uptake and using low-protein diet in the therapy; however, patients with hepatic insufficiency who suffer from decreased serum albumin level due to enhanced decomposition of body protein must take up a minimum maintenance level of protein (1.27 g/kg body weight/day) by all means. However, to patients who are intolerant of protein, even taking up this minimum maintenance level of protein is problematic. It is therefore necessary to develop a method for the treatment of hyperammonemia by other than low-protein diet [Akiharu Watanabe, Rinsho Kanfuzengaku (Clinical Study of Hepatic Insufficiency), pp. 297-307, 1994].
As a method for the treatment of hyperammonemia by other than low-protein diet, administration of lactulose and a nonabsorable antibiotic “neomycin” has heretofore been tried. In 1966, lactulose was first used in the treatment of hepatic encephalopathy (Bircher J. et al. Lancet 1:890-893, 1966) and its effectiveness (80-90%) was later verified by double blind test (Conn HO, et al. Gastroenterol. 72:573-583, 1977). Ever since that time, lactulose has been widely used in both prevention and treatment of hepatic encephalopathy which accompanies fulminant hepatitis and cirrhosis. Lactulose (4-O-β-D-galactopyranosyl-D-fructose) was made from lactose by E. M. Montgomery et al. in 1930 and it is a non-naturally occurring oligosaccharide composed of one molecule each of galactose and fructose. Nonabsorbable antibiotics such as neomycin have side effects (renal disorder and deafness), so the frequency of their use is comparatively low and lactulose has been considered “the drug of first choice” against hyperammonemia [Akiharu Watanabe, Rinsho Kanfuzengaku (Clinical Study of Hepatic Insufficiency), pp. 297-307, 1994].
Lactulose is believed to prevent or ameliorate hyperammonemia and hepatic encephalopathy by the following mechanism of action.
1) Lactulose promotes the growth of organic acid producing enterobacteria such as Bifidobacteria to lower the pH in the colon, thereby converting the ammonia in the intestine to the ionic form (NH4+) so as to suppress the absorption of ammonia: 2) Lactulose suppresses the growth of ammonia producing bacteria in the intestine so as to suppress ammonia production in it; 3) When carbohydrates are supplied as a source of energy, enterobacteria take up nitrogen compounds (e.g. urea and ammonia) and use them as raw materials for the synthesis of amino acids and proteins so that the ammonia level in the intestine is lowered [Attachment to LACTULOSE MATSU “NIKKEN”, Nikken Chemicals Co., Ltd., 1998 and Akiharu Watanabe, Rinsho Kanfuzengaku (Clinical Study of Hepatic Insufficiency), pp. 297-307, 1994].
The human digestive tract has no enzyme that decomposes lactulose into galactose and fructose, so it is held that lactulose is not absorbed into the small intestine but that it reaches the large intestine where it is utilized by enterobacteria to exhibit the various functions mentioned above.
In Japan, lactulose is commercially available as powder, syrup, dry syrup and jelly. The powder is usually administered orally daily equivalent dose to 18-40 g lactulose by patients with cirrhosis, it is usually divided in two or three portions and dissolved in cold or lukewarm water prior to use. The syrup should usually be administered as a 65% lactulose solution in a daily dose of 30-60 ml per adult which is divided in three portions.
However, it has been pointed out that lactulose has several defects. For example, lactulose has a lower enterobacterium proliferating effect than other oligosaccharides and must be administered in large amounts in order to obtain the above-described effects. However, lactulose is so sweet that taking it daily in large amounts is considerable pain to the subject. Further, taking large amounts of indigestable saccharides such as lactulose often causes diarrhea, which is another problematic side effect of lactulose.
In addition, lactulose is contraindicated against galactosemic patients (Attachment to LACTULOSE MATSU “NIKKEN”, Nikken Chemicals Co., Ltd., 1998). Lactulose preparations contain galactose (≦11%) and lactose (s 6%) and cannot be used in patients with galactosemia which is an inborn metabolic abnomally due to congenital deficiency of the enzyme for the galactose metabolic system. Patients with diabetes mellitus also require meticulous administration of lactulose (Attachment to LACTULOSE MATSU “NIKKEN”, Nikken Chemicals Co., Ltd., 1998). The galactose (s 11%) and lactose (≦6%) in lactulose preparations are metabolized to glucose, thereby elevating the blood sugar level is problematic for diabetic patients after decomposition and absorption. Care must also be taken when lactulose is used in combination with the antidiabetic drug α-glucosidase inhibitor (Attachment to LACTULOSE MATSU “NIKKEN”, Nikken Chemicals Co., Ltd., 1998). The α-glucosidase inhibitor inhibits the decomposition of carbohydrates in food, thereby lowering the absorption of glucose and hence is used in order to suppress the elevation of the blood sugar level after meal. The administration of the α-glucosidase inhibitor is known to induce side effects in the digestive system (e.g. abnormal fermentation with enterobacteria); since lactulose also promotes enterobacterial fermentation, using it in combination with the (t-glucosidase inhibitor presents a concern over enhanced side effects.
The following are documented side effects of lactulose from use against hyperammonemia.
Digestive organ: Diarrhea, occasionally accompanied by abdominal pain, borborygmus, bloat, anorexia, vomiting, etc.; if aqueous feces are caused, the administration should be reduced in quantity or suspended [Nihon Iyakuhinshu (Pharmaceuticals in Japan), ed. by Nihon Iryo Joho Center (Japan Medical Information Center), Yakugyojihosha 1997].
As described above, the use of lactulose as a therapeutic of hyperammonemia has been partly replaced by nonabsorbable antibiotics (e.g. neomycin); however, due to the many side effects they cause and since they fail to show the intended effects in many cases of actual use, the nonabsorbable antibiotics are no longer popular today.
Under these circumstances, it has been desired to develop therapeutic agents against hyperammonemia and hepatic encephalopathy that are safe (cause fewer side effects) and easy to take, which develop positive efficacy upon administration in small amounts and which can also be administered to patients with galactosemia and diabetes mellitus.